1. Field of the Invention
The present invention relates generally to devices incorporated into an image recording apparatus or the like used to form, e.g., print an image on a sheet of paper or other types of recording media, for transporting a recording medium, and in particular to improvements of such devices driving a belt to transport a recording medium.
2. Conventional Art
Printers and copiers are conventionally known apparatuses for forming, e.g., printing an image on a sheet of paper, film or other types of recording media, as disclosed for example in Japanese Patent Laying-Open No. 6-135613. Generally, such apparatuses employ electro-photography, ink-jetting and the like to form an image.
In electro-photography, a tonered image formed on a photoreceptor drum is transferred onto a recording medium to form an image on the recording medium. In ink-jetting, a printing head jets ink toward a recording medium to form an image on the recording medium.
The above two methods each employ a belt drive device as means for transporting a recording medium. Ink-jet printers using this belt drive device typically have a configuration, as described below:
As shown in FIG. 4, a belt drive device of this type includes a drive roller a, a subordinate roller b and a tension roller c, with an endless belt d engaged therearound. Drive roller a, connected to a drive shaft of a motor (not shown), receives the motor""s driving force and thus rotates. As drive roller a rotates, belt d in the figure runs in a direction A. Opposite to subordinate roller b, on an upper side in the figure, a pinch roller e is arranged to cooperate with subordinate roller b to pinch belt d. In a vicinity of subordinate roller b, a sheet feed cassette f is arranged, from which a recording medium (a sheet of paper) g is drawn and transported in direction A, pinched together with belt d between subordinate roller b and pinch roller e, as belt d runs.
A printing head h is arranged above belt d between drive roller a and subordinate roller b. The portion of belt d between drive roller a and subordinate roller b will be referred to as a span s. Printing head h is a linear head or a serial head. The linear head has a multitude of jet nozzles depending on the resolution of interest that are arranged across a printing width as required in a direction perpendicular to the plane of FIG. 4, e.g., 200 mm for a sheet of the A4 size. The serial head has several tens to hundreds of jet nozzles in direction A as shown in FIG. 4 and prints an image on recording medium g as it moves in a direction perpendicular to the plane of FIG. 4.
In printing an image, as the belt drive device is driven recording medium g is drawn from sheet feed cassette f, pinched together with belt d between subordinate roller b and pinch roller e and thus transferred in direction A.
For printing head h in the form of the linear head, recording medium g is continuously transferred, while the printing head""s nozzles jet ink appropriately to print an image on recording medium g.
For printing head h in the form of the serial head, recording medium g is initially transferred to the position at which printing head h is arranged.
When recording medium g has arrived there, belt d halts. Then printing head h jets ink through its nozzles as it moves in the direction perpendicular to the plane of FIG. 4, and it thus prints an image. When printing head h has arrived at one end of recording medium g, belt d again starts to run and then stops after recording medium g has been moved by a predetermined distance. Then printing head h then again moves in the direction perpendicular to the plane of FIG. 4 and thus prints an image. Thus, the print operation by printing head h and the recording-medium transport operation by the belt drive device are alternately provided to print an image on recording medium g.
One exemplary device of this type also includes a platen chamber i arranged on a back side of the belt""s span s existing between drive roller a and subordinate roller b. Platen chamber i can aspirate recording medium g on belt d so that recording medium g does not displace and is thus transported satisfactorily. To do so, platen chamber i has an upper surface provided with multiple aspiration holes j. Similarly, belt d is also provided with multiple aspiration holes (not shown). When platen chamber i is driven or a negative pressure is produced, an aspiration force is produced at each aspiration holes of belt d. Thus, belt d aspirates recording medium g to prevent any positional displacement of recording medium g while transporting recording medium g.
The conventional belt drive device configured as above, however, tends to transport recording medium g by a distance larger than a predetermined distance. If such event occurs, a difference would be introduced between the driveability of belt d and the transportability of recording medium g. Thus, recording medium g would slide relative to belt d and thus be positionally displaced from a predetermined position. As a result, recording medium g would be transported with less precision or worse it would come off belt d and contact printing head h or divert from a transporting path and jam up the machine of interest, resulting in an unsatisfactory print operation. As such, recording-medium transport devices using such conventional belt drive device are still disadvantageous in transporting recording medium g.
The present inventors have studied what introduces a difference between the driveability of belt d and the transportability of recording medium g and have found that such difference is introduced by the following event:
More specifically, it has been found that, as shown in FIG. 5, when recording medium g is introduced between subordinate roller b and pinch roller e, in a vicinity of the portion pinched by rollers b and e, which will be referred to as a nip hereinafter, belt d has a front surface running at an temporarily increased rate, which introduces the difference between the driveability of belt d and the transportability of recording medium g. More specifically, belt d around subordinate roller b has an expanding front surface and a contracting back surface. Furthermore, as indicated in FIG. 5 by a broken line, belt d has a layer approximately intermediate as seen in a direction of the belt""s thickness that neither expands nor contracts.
The front surface of belt d around subordinate roller b, expanding, runs at a local, higher rate than the belt""s back surface and intermediate layer.
For example, if xcfx89 represents a rate at which subordinate roller b revolves, rn represents a distance from the center of subordinate roller b to the intermediate layer of belt d, and xcex94r represents a distance from the intermediate layer of belt d to the front surface of belt d, then belt d around subordinate roller b has a front surface running at a rate Vin:
Vin=(rn+xcex94r)xc3x97xcfx89xe2x80x83xe2x80x83(1).
Furthermore, the intermediate layer, neither expanding nor contracting, runs at a rate Vn:
Vn=rnxc3x97xcfx89xe2x80x83xe2x80x83(2).
As such, the belt""s front surface runs faster than the belt""s intermediate layer by the difference between the two rates, i.e., xcex94rxc3x97xcfx89. While the front surface of belt d around subordinate roller b runs faster than the intermediate layer of belt d therearound, a front surface of belt d not around subordinate roller b, e.g., that of belt d opposite to printing head h, no longer expands and thus runs at a reduced rate and thus the same rate as the belt""s intermediate layer.
Thus, belt d has a front surface running faster at subordinate roller b (at a rate V1 in FIG. 5) than at printing head h (at a rate V2 in FIG. 5). Because of such difference in rate, in FIG. 5 at the nip there would work a force which would transport recording medium g on a front surface of belt d at a rate greater than that of the intermediate layer of belt d. That is, recording medium g is transported farther at the nip than at printing head h. As such, recording medium g cannot contact the front surface of belt d, with a result that recording medium g and belt d slide relative to each other or recording medium g comes off the front surface of belt d. Consequently, recording medium g would be brought into contact with printing head h or divert from a transporting path and jam up the machine of interest.
One object of the present invention is to achieve an image formation, wherein a variation in driveability between a portion of a belt engaging and contacting a roller and a portion of the belt out of contact with the roller does not have any effect on a recording medium being transported, to reliably transport the recording medium to form on the recording medium an image of high quality as well as prevent the recording medium from diverting from a transporting path and thus achieve reliable image formation.
In order to achieve the above object, the present invention provides a recording-medium transport device configured to have a plurality of rollers with an endless transport belt engaged therearound and having a portion corresponding to a span for transporting a recording medium, having a transporting side for carrying a recording medium on which image formation means forms an image. The recording-medium transport device is characterized by a pair of pinch rollers contacting a transporting side of the recording-medium transporting span and a back side of the span, respectively, while maintaining the span flat, to pinch the recording medium together with the endless transport belt and thus transport the recording medium toward the image formation means.
As such, the endless transport belt has a transporting side bearing thereon and thus transporting a recording medium pinched together with the endless transport belt by the pair of pinch rollers. The pair of pinch rollers pinch a portion of the belt out of contact with the plurality of rollers. Thus the pair of pinch rollers pinch a portion of the endless transport belt which does not have a curve. As such, the belt does not have an expanding surface and in a vicinity of the pair of pinch rollers the belt has a surface running at a constant rate while transporting the recording medium, on which an image formation means forms an image. Thus, there would not be a difference introduced between the driveability of the belt and the transportability of the recording medium, nor would the recording medium slide relative to the belt or be positionally displaced from a predetermined position. Thus the recording medium can be transported satisfactorily, with high precision. As such, the recording medium does not come off the belt or contact a printing head or the like or divert from a transporting path or thus jam up the machine of interest.
In one embodiment of the present invention, the plurality of rollers with the endless transport belt therearound correspond to a drive roller and a subordinate roller and rotating and thus driving the drive roller allows the endless transport belt to run so that the endless transport belt has an upper surface transporting the recording medium, while the pair of pinch rollers includes an outer pinch roller in contact with the upper surface of the endless transport belt and an inner pinch roller in contact with a back surface of the endless transport belt, wherein the drive roller, the subordinate roller and the inner pinch roller are arranged to have their respective upper edges aligned in a single plane.
In another embodiment, as is similar to the above, the endless transport belt has an upper surface adapted to transport a recording medium, while on a back side of the recording-medium transporting span of the belt there is provided aspiration means producing a force to aspirate the recording medium on the endless transport belt, wherein the pair of pinch rollers includes an outer pinch roller in contact with the upper surface of the endless transport belt and an inner pinch roller in contact with an back surface of the endless transport belt and are arranged between a roller with the belt engaged therearound and the aspiration means and wherein the drive roller, the subordinate roller, the aspiration means and the inner pinch roller have their respective upper edges aligned in a single plane.
As such, there can be implemented a configuration for pinching the recording-medium transporting span of the endless transport belt between the pair of pinch rollers while maintaining the span flat. In particular, if the aspiration means is also provided, its aspiration allows the recording medium to be aspirated on the endless transport belt to reliably prevent the recording medium from sliding relative to the belt and having a positional displacement from a predetermined position, thus more reliably transporting the recording medium.
In the present invention, the recording-medium transport device preferably has the pair of pinch rollers generally equal in outer diameter to allow the endless transport belt to have a front surface and a back surface contacting the paired pinch rollers in the same condition. This ensures that the endless transport belt pinched by the pair of pinch rollers is flat.
In another embodiment the recording-medium transport device includes registration means starting to run the endless transport belt with a recording medium having its preceding edge abutting between the endless transport belt and an outer pinch roller of the pair of pinch rollers in contact with an upper surface of the endless transport belt, to register the recording medium so that a side of the recording medium that corresponds to the recording medium""s preceding edge extends orthogonal to a direction in which the endless transport belt runs, for transporting the recording medium.
With the recording medium having its preceding edge abutting between the pair of pinch rollers and the endless transport belt, the pair of pinch rollers and the endless transport belt, in contact with each other, form therebetween a line (parallel to each pinch roller""s rotation axis) parallel to the preceding edge of the recording medium. Thus the recording medium with its preceding edge satisfactorily registered in position passes through the pair of pinch rollers.
Running the endless transport belt with the recording medium thus registered allows the recording medium to be transferred, registered on the endless transport belt, with a longitudinal direction of the preceding edge of the recording medium orthogonal to a direction in which the endless transport belt runs.
In the present invention, preferably the recording-medium transport device is incorporated into a recording-medium transport system of an image recording apparatus to transport a recording medium on the recording-medium transporting span of the belt and pass the recording medium through the pair of pinch rollers before the image formation means form an image on the recording medium. As such, there can be obtained a specific application of the recording-medium transport device of the present invention, allowing the image recording apparatus to form an image of high quality.